Frequency multiplier



July 2s, 1942. w. BEM. 2,291,366

FREQUENCY MULTIPLIER Filed May 8, 1941 MJUSTAELE 50x/emol: FMVDHMSNTHL deff@ 5 l aff/1:)

t INVENTOR frlr ,enz

Patented July 28, 11942 2,291,366 FREQUENCY MULTIPILIER' Werner Benz,

Berlin-Steglitz, Germany, assignor to General Electric Company, a corporation of New York Application May 8, 1941, Serial No. 392,470 In Germany April 10, 1940 8 Claims.

To obtain a denite harmonic of a higher order from a given fundamental frequency, general practice in the earlier art has been to distort the fundamental wave in a non-linear quadripcle, the particular harmonic that is desired being filtered from 'the multiplicity or spectrum of harmonies resulting from such distortion by the aid of a suitable oscillation or tuning circuit or filter. This procedure has the disadvantage that for each such harmonic a distinct oscillation circuit or. filter is required, and that, for instance, upon a change of the fundamental wave the tuning of the oscillation circuit provided for the desired harmonic must be altered or the filter be replaced.

The present invention, which overcomes these disadvantages, ,is concerned with a circuit adapted to secure the third harmonic of a fundamental wave by the aid of a non-linear four-terminal network designed to distort the said frequency. For this purpose there is employed a quadripole arranged in a bridge and known in the prior art as an amplitude lter, the balancing of these quadripoles being chosen so that a characteristic connecting the output potential u2 with the input potential u1 results, which, inside a certain drive range, fulfills roughly the function uz=% ku1-ku13, where lc is a constant.

The invention oifers the following advantages:

(l) If the requirements respecting the purity of the third harmonic are not unduly severe (in other words, if a non-linear harmonic distortion of is admissible), the oscillation circuit or iilter to separate the said harmonic is dispensable. (2) But, if the duty requirements respecting the purity of the third harmonic are very severe, the requisite filter means are far less elaborate than is true of the arrangements heretofore known, since the third harmonic appears in far stronger form than the fundamental wave and the undesired harmonics. (3) If demands regarding purity of the .third harmonic are not excessively high, this harmonic is obtainable in.- side a very wide frequency band which is practically a function only of the natural or self capacitance of the non-linear elements forming a part of the distorting means, regardless of changes of the fundamental frequency.

In the drawing: A

Fig. 1 shows. a circuit embodying the invention; f

Fig. 2 shows the current-voltage characteristic of thefindividual rectiiiers used in Fig. 1; and

Figs.f3.a;- 3b and 3c show curves illustrating the relation of input to output voltage of the circuit of Fig. 1. Fig. 3a shows the relation between input and output voltage for the circuit of Fig. 1,

. curve I being for the case where resistor R of lows Fig. 1 is chosen substantially equal to the asymptotic value of effective rectifier resistance, and curve II is for the case where resistor R of Fig. 1 is chosen substantially greater than said limiting value. Fig. 3b shows a cycle of alternating voltage input to Fig. 1 of amplitude` so .related to curve II of Fig. 3a that the output voltage of Fig. l is given by curve IV ofy Fig. 3c. lIn Figs. 3b and 3c the correspondirg points in time are indicated by the same numerals. l

Fig. l shows a bridge arrangement known in the prior art for use as an amplitude limiter,

though the same is here used as a distorting j quadripole. The bridge contains in opposite arms two like ohmic resistances R or two resistances G non-linear in nature and of the same resistance behavior, the said non-linear resistances consisting of the parallel connection of two similar rectiers, say, selenium rectiers, of opposite forward or low resistance direction. One of the diagonals of this bridge scheme is fed with the fundamental frequency potential u1 and the other one delivers the4 consumer or load potential uz; that is, the third harmonic of the fundamental wave.

To further explain the mode of action of this circuit organization, reference shall in what folbe made to Figs. l2 and 3 of the drawing.

Fig. 2 shows the current i-voltage u, characteristic of a selenium rectifier of lthe kind to be used, for instance, for the non-linear resistances G. Extending the straight portion of this characteristic to the point of intersection with the abscissa axis, then the cotangent to the angle of inclinationof this straight line in reference to the abscissa axis represents an effective resistance Rgzcot a. If, then, one chooses the resistance R=Rg, the arrangement shown in Fig. 1, as known from the art, acts like an amplitude limiter. or filter, the dependence of the instantaneous values ofthe output potential u2 upon those of the input potential u1 being illustrated by graph I, Fig. 3a.

Making It greater than Rg, then the previously horizontal portions of graph I will droop so that, for a definite value of R, there results, for instance curve II, Fig. 3a. Now, by judicious choice of the non-linear resistances G and the linear resistances R, it is. always possible to rind a graph II so that, for a definite amplitude of a purely sinusoidal input potential u1 (see curve 5 III, Fig. 3b) an output potential un results which Suppose, for the sake of deiiniteness, that an' input voltage U1 is applied to the bridge of Fig. 1 t

with such polarity as to make the upper input terminal positive. As the input voltage slowly grows from zero, there will be a slowly growing output voltage U2, such that the left hand output terminal is positive. This is because rectifiers G have very high resistances to small applied voltages compared with resistors R. When the input voltage reaches a certain strength, however, the ratio. of voltage drop across each of theconducting rectiflers to the current therethrough becomes equal to resistance R, so that the bridge is in balance. Thus, the output voltage has started from zero, risen to a maximum, and returned to zero, all while the input voltage is still increasing. Upon a further increase of input voltage, the rectier resistance becomes less than R, so that the output voltage starts becoming negative and continues to become more and more negative up to the point where the input voltage reaches its maximum value. The output voltage is thus represented by the first three-quarters of a cycle of the curve IV, which occurs during the first quarter cycle of curve III which represents the input voltage. As the input voltage decreases from its maximum, the output voltage retraces the series of values it had previously taken on, so as to complete the next three-quarters of a cycle of curve IV coincidentally with the input voltage completing its first half cycle by falling to zero, shown at point 3 of curve III.

The negative half of the wave oi' input voltage will be symmetry of the bridge produce the negative of the iirst three half cycles of curve IV, thus carrying this curve up to point 6. It will be seen that the application of one full alternating cycle of suitable amplitude to the-input terminals of Fig. 1 will result in three full cycles oi alternating output voltage and the important thing about this is that no resonant circuit is relied upon to produce or select out this triple frequency output.

In what follows, the operation of the circuit organization of ,the invention shall be mathematically investigated by calculation by,reference to these curves.

Suppose the characteristic II of Fig. 3, for a definite range, satises the equation uz=K' u1-Ku1f (-1) Suppose further that u1=U sin a (2) where a=lwt| dnd u=21rf (3) from (l) and (2) there results:

u2=K"U Sin a-KUs S111s a (4) or l FK/U sin a-KUG sin asin sa) (s) Qnd U KKUI sin HKUI sin 3a (6) for the special case l In other words, what is obtained in this case is nothing but the third harmonic of the fundamental wave.` Making the amplitude of thel latter U=l, then andthus for characteristic II, Fig. 3, there results this equation:

,-Ka-Ku.' (9) Now, under practical conditions the graph represented by the preceding equation cannot be strictly realized. However, by suitable choice of the linear and the non-linear resistances of the distortion means. at least inside a definite range, such a close proximation to this curve may be obtained that the desired result will be obtainable to an adequate extent.

It will be understood that the invention is by no means confined to the bridge arrangement in the form of a quadripole as here illustrated and described; on the contrary, in lieu of this arrangement any other bridge schemes known in the art of amplitude iilters could be used. Inside the bridge, it is, of course, not necessary that the resistance ratio of opposite arms of the bridge should be as 1:1. In certain circumstances it may also prove to be of advantage to impress the non-linear resistances with a negative biasing potential. Moreover, there is a chance to use a plurality of such distorter arrangements each adapted to obtain the third harmonic and connected in series with the result that the 9th, the 27th and higher harmonics are obtained.

The invention is adapted to be used with par-V ticularly great -advantage in connection ywith heterodyne buzzers or beat frequency oscillators. To the end of avoiding the troublesome pull-instep eect arising in these devices, it is known from the earliest art to cause the second and third harmonic of the fundamental waves oi' the two transmitters to enter in beat relations. In such schemes previous practice has been to vary the transmitter furnishing the second harmonic. while that of the other transmitter was unvaried. As a consequence, upon making such change in the frequency, it has always been necessary' to change also the tuning of the oscillatory circuit tuned to the second harmonic. Where the invention is used for oscillators or buzzers -of the said kind, the transmitters whose second harmonic is employed, is left unchanged, while the other transmitter is made variable, the third harmonic of this transmitter being produced by adopting a circuit arrangement as here disclosed. In this case, variation of tuning of the oscillatory circuit that has heretofore been necessary is dispensable since in practicing the invention no filter or oscillation circuits are required to obtain the third harmonic.

What is claimed is:

1.. A circuit for producing the third harmonic of a fundamental sinusoidal wave, comprising a non-linear quadripole in the form of a fourterminal network designed to distort the fundamental wave, said network constituting a bridge, a source of alternating current of fundamental contrary forward directions.

frequency coupled to one pair of said terminals of said network, and an output circuit coupled to the other pair of said terminals, the arms of said bridge having such values that there results a characteristic connecting the output potential and the input potential which, within a definite operating range, 4satisfies roughly the function where lc is a constant/,a1 is the input potential and u2 is the output potential.

2. Circuit organization as claimed in claim l, with the characteristic feature that the distorting quadripole consists of abridge containing in one pair of opposite arms two equal ohmic re-v sistances and in the other pair of opposite arms 4. In combination, a device having output voltage which changes sign a number'of times as an instantaneous magnitude of input voltage increases from zero to a predetermined amount and whose output voltage varies approximately sinusoidally with respect to said input voltage, means to impress alternating input voltage on said device, means for adjusting the amplitude of the alternating input voltage to cause the output of the device to change sign a desired nurnber of times per cycle of input voltage, whereby the output voltage frequency is equal to the inwhere l'c is a constantyui is the input potential,

l and u2 is the output potential, means for impressing a voltage wave of fundamental frequency on said input terminals, and means for utilizing the third harmonic wave produced between said output terminals.

6. A system for obtaining an odd harmonic of a fundamental wave comprising a first third harmonic frequency vproducer in the form of a non-linear four-terminal network designed to distort the fundamentalwave, said network constituting a bridge, a source of alternating current of fundamental frequency coupled to one pair of said terminals of said network, and an output circuit coupled to the other pair ofsaidtermihals,` the arms of said bridge having such values that there results a characteristic.connecting the output potential and the input potential which, within a definite operating range, satisfies roughly the function4 where 7c is a constant,4 u1 is the input potential, and u2 is the output potential, and a second similarly arranged third harmonic frequency producer four-terminal network, and means for connecting said two four-terminal networks in series, whereby there is obtained a harmonic which is an odd multiple of said third harmonic.

7. In a frequency triplersystcm for producing an output voltage whose frequency is three times n the input frequency, a device for producing an put voltage frequency multiplied by one-half said desired number and the out-put voltage is substantially sinusoidal.

5. A frequency multiplier for producing a third harmonic of a fundamental wave comprising a Wheatstone bridge arrangement having a pair of input terminals and a pair of output terminals connected toy conjugate points, a pair of ohmic resistors in opposite arms of the bridge, the other arms of the bridge being each composed of a pair of oppositely poled rectiflers connected in parallel, the ohmic resistors-having such values that there results a characteristic connecting the output potential and the input potential which within .a definite operating range satisfies roughly the equation output voltage which changes sign at least once as the instantaneous input voltage increases from zero to a predetermined value, means for applying an alternating input voltage to said device, and means for adjusting the amplitude of said input voltage to cause the output voltage of said device to change sign six times per cycle of input voltage.

8. In combination, a device having an output voltage vwhich is zero for zero input voltage,'and which for increasing input voltage of one sense said output voltage increases at first, passes through a maximum, decreases to zero, reverses in sign and increases in magnitude, while for an input voltage of the other sense the ouput voltage varies similarly but with its sign opposite to the output voltage rst described at corresponding magnitudes of input voltage, means to impress alternating input voltage on said device, means for adjusting the amplitude of the alternating input voltage to cause the output of the device to change sign a desired number of times per cycle of input voltage, whereby the output voltage frequency is equal to the input-voltage frequency multiplied by one-half said desired number and the output voltage is substantially sinusoidal.

WERNER BENZ. 

